Due to the physical characteristics of a recording device, the (d, k)-constrain of the Run-Length Limited (RLL) modulation used with a particular device have values within a particular design range. To design a modulation algorithm with certain (d, k)-constrain, the coding rate of this modulation algorithm has a theoretical maximum value. If the actual coding rate of this modulation algorithm is more closed to the theoretical maximum value, it is more efficiency. But we know that the coding rate will influence the capacity of recording medium. So that, many existing RLL modulation algorithms in this field aim to increase code efficiency. In general, these high efficiency modulation algorithms will reduce the number of selectable codeword and the available choices of paths, and then reducing the ability of DC-free control.
To improve the ability of DC-free control, one possible method is to use a look-ahead path modulation, which calculates several data words, then determines the most suitable codewords for modulation corresponding to the foremost data word; therefore, the longer the pre-calculated look-ahead path, the better the ability of DC-free control. However, this kind of modulation increases calculations exponentially when the length of the look-ahead path increases, thus reducing overall modulation speed.
Because a high efficiency modulation requires the look-ahead path modulation to increase the ability of DC-free control, and how to reducing the calculations required for look-ahead path modulation and increasing the modulation speed has become a major focus for many research projects. U.S. Pat. No. 5,917,857 provides a recording medium modulation apparatus for EFM and EFMPlus modulation, but it is based on DVD specifications and is only suitable for a specified DC-free control method, not for a fixed length look-ahead path modulation.
Because the look-ahead path modulation has become very popular, a more efficient look-ahead algorithm that can be implement in the hardware is needed. How to increase the execution efficiency and to decrease the cost of hardware by reducing the calculation in the look-ahead path modulation procedure is very important.